Safety device for preventing electric shock

ABSTRACT

A ground fault interrupter circuit utilizes the potential difference between the neutral supply line and ground to sense faults. An ordinary resettable magnetic circuit breaker is used, but delay circuitry prevents immediate reenergization of the breaker coil resulting from a surge when resetting is attempted.

United States Patent Scanlan [151 3,676,738 [45] July 11,1972

1541 SAFETY DEVICE FOR PREVENTING ELECTRIC SHOCK [72] Inventor: WarrenF. Scanlan, Philadelphia, Pa.

[73] Assignee: United Kingdom Atomic Energy Authority,

London, England [22] Filed: March 24, 1970 [21] Appl. No.: 22,152

[52] US. Cl. ..3l7/l8 R, 317/49 [51] Int. Cl. ..H02h 3/16 [58]FieldofSearch ..3l7/49, 18 D, 18R, 27R

[56] References Cited UNITED STATES PATENTS 3,329,913 7/1967 Camp..335/38 LINE T0 LOAD 3,259,802 7/1966 Stecn ..3l7/18 3,319,123 5/1967Scanlan ..317/l8D Primary Examiner-James D. Trammell Attorney-Smith,Harding, Earley & Follmer [5 7] ABSTRACT A ground fault interruptercircuit utilizes the potential difference between the neutral supplyline and ground to sense faults. An ordinary resettable magnetic circuitbreaker is used. but delay circuitry prevents immediate reenergizationof the breaker coil resulting from a surge when resetting is attempted.

8 Clairm, 2 Drawing Figures BACKGROUND OF THE INVENTION This inventionrelates to safety devices, and particularly to a class of electricalsafety devices known as ground fault interrupters."

Various circuits have been used in the past for the prevention ofphysical injury resulting from electric shock,'and the most successfuldevices, so far, have been those which effect interruption of currentwhenever a ground fault is sensed. A ground fault will occur when thebody of a person comes into contact with an energized conductor, eitherbecause the body is in direct contact with ground, or because thecapacitance between the body and ground is sufficient to conduct adetectable current. Different devices have been used for detecting suchfaults. For example, one scheme utilizes a differential transformerhaving a first winding connected in series with one side of the line,and a second winding in series with the other side of the line,-inbalanced relation with the first winding. A third winding becomesenergized whenever a ground fault occurs because the ground faultprevents the current in the first winding from being equal to andbalancing the current in the second winding.

Another scheme, as illustrated in my U.S. Pat. No. 3,319,123, issued May9, I967, utilizes an isolation transformer in conjunction with a balancecircuit which becomes unbalanced whenever a ground fault occurs.

In these circuits, means are provided by which the line current isinterrupted as rapidly as possible whenever a fault is detected. Themore rapidly the current is interrupted, the less electrical energy isabsorbed by the person causing the fault.

One difficulty with interrupters utilizing differential transformersresults from the sensitivity of differential transformer cores totemperature changes. In devices utilizing differential transformers,temperature changes may render the sensitivity either too low or toohigh.

Interrupters utilizing transformers which isolate the load from the lineare bulky and expensive.

Ground fault interrupters require a breaker to interrupt line currentwhen a fault is detected. One problem arises because of a safetyrequirement that circuit breakers be constructed so that their purposecannot be defeated by mechanically holding the reset button in aresetting condition. This might be done, for example, by taping thereset button down. Most circuit breakers are constructed so thatenergization of the magnetic coil which effects interruption will effectinterruption even if the reset button is in the depressed condition. Itis then necessary to release the reset button and depress it again underconditions such that the coil will not be energized. When these breakersare utilized in ground fault interrupter circuits, an attempt to resetthe breaker to restore current to a load may produce a surge which maybe sensed as a ground fault. This will effect operation of the breaker,so that it is impossible to restore current to the load.

Another problem exists with ground fault interrupter units which aredesigned for permanent installation. If any part of the unit breaksdown, line current may be interrupted for a long period of time until arepair can be eflected.

SUMMARY OF THE INVENTION In accordance with this invention, nodifferential transformer is used, and no isolation transformer capableof handling the entire load current is necessary. Advantage is taken ofthe potential difference between the neutral side of a supply line andground to effect a detectable change in current in a circuit branch whenground fault occurs.

The circuit in accordance with the invention makes use of a resettable,electro-mechanical breaker of the kind constructed so that it cannot bedefeated by holding the reset button down. Delay circuitry is providedto prevent immediate reenergization of the energizing coil of thebreaker when resetting is attempted. This prevents electrical surges,which may be interpreted by the sensing circuitry as ground faults,

from eflecting immediate reopening of the breaker whenever resetting isattempted. The circuitry, however, does not defeat the purpose of thespecial breaker construction, since even if the reset button is helddown, the breaker will operate when a fault occurs.

One object of the invention, therefore, is to provide a safety devicefor preventing electrical shock which does not require a differentialtransformer, a bulky isolation transformer or any other specializedtransformer or balancing circuit.

Another object of the invention is to provide a safety device forpreventing electric shock which utilizes a resettable breaker, but whichcannot be defeated by holding the resetting control.

Another object of the invention is to provide a safety device forpreventing electric shock in which the sensing circuitry is in a modularunit which may be readily replaced in the event of a malfunctionresulting in an undesired and unnecessary interruption of current, andin which current is not interrupted for a long period of time as aresult of such a malfunction.

Another object is to provide a safety device for preventing electricshock which is reliable, fail-safe, highly sensitive and rapid in itsoperation. Other objects will be apparent from the followingdescription, read in conjunction with the accompanying drawing.

BRIEF DESCRIPTION OF THE DRAWING FIG. 1 is a schematic diagram of thesafety device in accordance with the invention; and

FIG. 2 is a side elevation showing the mechanical details of aresettable circuit breaker suitable for use in the circuit of FIG. 1.

DESCRIPTION OF THE PREFERRED EMBODIMENT The circuitry, as shown in FIG.1, is in the condition which it would be in during normal operation,with current being delivered to a load, and with no ground faultoccuring.

In FIG. 1, pennanently installed circuitry is indicated generally at 2,and reference numeral 4 indicates sensing and control circuitry,preferably provided in a plug-in module, adapted to be connected to thepermanent circuitry by the engagement of a multiple plug 6 on the modulewith a socket 8.

An alternating current supply line, which would typically carry voltscomprises a high" side 10, and a neutral" side 12. In most terminalboxes through which electricity is supplied to the home, the neutralside 12 of the line is at a potential which is different from ground.That is, with a nominally 120 volt (rrns) line, the voltage which willbe measured between the neutral side and ground will be typically 15volts, while the voltage measured between the high side and ground willbe typically I05 volts.

In the permanent wiring, terminal 14 of socket 8 is connected throughwire 16 to ground, preferably to a water pipe.

' The neutral side 12 of the line may be connected directly to a load18, e.g., an appliance or a group of appliances, through line 18.

The high side 10 of the line may be connected to a load through line'20, but line 20 is connected to line 10 through the contact element 22of a breaker 24. Breaker 24 has a winding in the form of a solenoid 26,which is mechanically arranged to open contact member 22 to effectinterruption of the circuit in the high side of the line when itreceives a signal through lines 28 and 30.

Breaker 24 is of the resettable type, and is preferably of the kind inwhich interruption of current can take place even though the resettingcontrol is held. A typical breaker of this type is disclosed in detailin U.S. Pat. No. 3,329,913, issued July 4, 1967 to W. W. Camp.

The typical breaker, as shown in FIG. 2, comprises a solenoid 25 and apair of current-carrying contacts 27 and 29, the latter being a movablecontact. Contact 29 is carried by movable arm 31, pivoted at 33. Aresetting handle 35 is operatively connected to arm 31 through acollapsible toggle mechanism or latching system 37. The toggle mechanismis connected to arm 31 through pin 39. Armature 41 is pivoted at 43 andarranged to be operated by solenoid 25. At one extreme of the armature,there is provided a trip member 45 which is able to engage trip arm 47when the solenoid is energized and the contacts are closed.

Thus, when the solenoid is energized, armature 41 is pivotedcounterclockwise about pivot 43, causing the trip member 45 to pivot tothe right and to trip arm 47 forming part of a latch assembly which,when the toggle mechanism 37 has been moved to the contacts closedposition, is juxtaposed with the trip member 45 whereupon togglemechanism 37 collapses under the pressure of a collapsing spring 49.

' Handle 34 is shown in the contacts closed condition. However, becausethe connection between the handle and the contacts is through acollapsible toggle assembly, it is possible for the contacts to beopened and current to be interrupted upon energization of the solenoid,even though the resetting handle is held in the position indicated.

Returning to FIG. 1, lines 28 and 30, which receive the signal foreffecting interruption by the breaker are connected respectively toterminals 32 and 34 of socket 8.

The primary winding 36 of a transformer 38 is connected between theneutral side 12 of the line and to the high side through contact member22. The purpose of this transformer is to supply operating current forthe sensing circuitry, and its secondary voltage and current ratingswill depend .on the requirements of the sensing circuitry. Secondarywinding 40 is connected between terminals 42 and 44 of socket 8. Theneutral side 12 of the line is connected directly to terminal 46 ofsocket 8, and the high side 10 of the line is connected, through contactmember 22, to terminal 48 of socket 8.

The permanently wired apparatus described so far may be wired into theterminal boxes in which electric current enters the home or buildingwhich is to be protected. However, in order to avoid interruption of theoperation of a large number of appliances in a building or home when afault occurs, several breakers 24 may be wired in different parts of thehome or building to localize the current interruption which will resultfrom the occurence of a fault. The breaker may even be wired into anindividual appliance, for example, a washing machine.

The circuitry in the sensing module will now be described. Terminal 50of plug 6, which is connectable to ground through terminal 14 of socket8 is connected through a resistor 52, and an inductor 54 to terminal 53of plug 6. Terminal 56 is connectable, through terminal 46 of socket 8,to the neutral side 12 of the line. Resistor 52 and inductor 54constitute a circuit branch which conducts current resulting from thepotential difference between the neutral side of the line and ground.The current in inductor 54 produces a magnetic field, the strength ofwhich depends on the magnitude of the current in this branch.

Preferably, inductor 54 is part of a magnetically shielded unit 55 whichconsists of inductor 54, a permanent magnet 56 and a sealed magneticallyoperable reed switch 58. The three elements in unit 55 are arranged sothat permanent magnet 56 maintains the contacts 60 and 62 of the reedswitch in a closed condition except when the magnetic field of inductor54 increases in strength beyond a predetermined limit. The reed switchis arranged in proximity to inductor 54 so that an increase in themagnetic field of the inductor will have this effect. The resistor 52and inductor 54 should be selected, and the elements in unit 55 arrangedso that a current of about 3 ma. in resistor 52 will result in openingof the contacts of the reed switch.

Capacitor 64 is connected in parallel with inductor 54 to cause themagnetic field of inductor 54 to vary through a greater range for agiven change in the current in resistor 52. Line 66 is connected toterminal 68 of plug 6, which is connectable through terminal 42 ofsocket 8 to one side of the secondary winding 40 of the transformer 38.Capacitor 70 is connected between inductor 54 and line 66 for thepurpose of preventing chattering of contacts 60 and 62 of the reedswitch, which may cause television interference.

While one side of the secondary winding of the transformer isconnectable to line 66, the other side is connectable through tenninal44 of socket 8 and terminal 72 of plug 6 to line 74. Line 74 isconnected through the winding of AC relay 76, and through reed switch 58to line 66. Line 74 is also connected through diode 78, resistor and thewinding of DC relay 82 to line 66. Capacitor 84 is connected across thewinding of relay 82.

Relay 76 has a set of normally closed contacts 86, that is, contactswhich are closed when the relay coil is in an unenergized condition.Relay 82 has a set of normally open contacts 88. Contacts 86 and 88 areconnected in series between terminals 56 and 90 of plug 6. Thesecontacts, therefore, provide a path for the energization of solenoidwinding 26 of the breaker from neutral line 12, through terminals 46 and53, through contacts 88 and 86, tenninals 90 and 34 and line 30. Thereturn path is through line 28, terminal 32 of socket 8, terminal 92 ofplug 6, line 94, terminal 96 of plug 6, terminal 48 and line 98 to thehigh side of the line through the breaker contact member 22.

The operation of the safety device will now be described. A

fault occurs when the body of a person comes into contact with line 20.The body is either in direct contact with ground, or has a capacitanceto ground. Therefore, a current is conducted from line 20 (the highside), to ground, through the body or through the body and itscapacitance to ground. The conduction of current through the body raisesthe potential of line 16 relative to the neutral side of the line.Consequently, the current through resistor 52 and inductor 54 increases,increasing the magnetic field of the inductor. When this occurs, themagnetic field of the inductor becomes sufficient to oppose the field ofpermanent magnet 56, and the contacts 60 and 62 of the reed switch comeapart.

The opening of the reed switch contacts disconnects operating current tothe winding of relay 76, and relay contacts 86 close. Since contacts 88of relay 82 are also closed at this time, a path is provided forenergization of winding 26 of the breaker 24. The breaker contacts openand remain open, deenergizing line 20. This operation takes place veryrapidly because of the rapid operation of unit 55 in response to aground fault occuring in line 20. Capacitor 64 aids this rapid response.

The operation is also fail-safe in that the winding of relay 76 must beenergized continuously for normal delivery of current to the load.

As mentioned previously, when a breaker is reset to restore current to aload, a surge can occur which might be interpreted by the fault detectoras a ground fault. Relay 82 and its associated circuitry prevent thedelivery of an interruption signal to winding 26 as a result of such asurge. Before resetting of breaker 24 is attempted, and while contacts22 are open, contacts 86 are closed and contacts 88 are open.

When resetting takes place, contacts 22 are closed, and contacts 86 mayremain closed momentarily, but for a sufficiently long time that aninterruption signal would be produced were it not for the fact thatcontacts 88 are open. Contacts 88 do not close until capacitor 84 hasbuilt up a sufficient DC charge to operate the winding of relay 82.Elements 82, 84, 80 and 78 constitute a time-delay relay, the delaybeing determined primarily by the values of resistor 80 and capacitor84. Contacts 88 only close after contacts 86 are finally opened.Consequently, no interruption signal is delivered to the breaker winding26.

Plug 6 and socket 8 allow disconnection of the sensing module 4, andimmediate replacement by a spare module. During the period of timebetween removal of one module and replacement by another, current can bedelivered to the load, but no interruption will be effected if a faulttakes place.

In practice, satisfactory results have been obtained where resistor 52is a IOOK-ohm resistor, capacitor 64 is a 15 microfarad capacitor,capacitor 70 is 0.25 microfarads,

capacitor 84 is 100 microfarads, resistor 80 is lK ohms, and transformer38 is a 1:1 transformer having a l.5 ampere secondary. The value ofinductor 54 depends on the strength of magnet 56, the characteristics ofreed switch 58, and the relative relationships of the elements in unit55. Satisfactory results have been obtained, however, where permanentmagnet 56 is a 32-gauss magnet, and inductor 54 is a lK-ohm coil.

Numerous modifications may be made to the circuitry described above.

For example, the means for conducting a current between the neutral lineand ground, while preferably a branch comprising a resistor and aninductor in series as shown, may consist of conventional transistorcircuitry for amplification of the current change caused by the fault.

The circuitry which prevents voltage surges from causing tripping of thebreaker when resetting is attempted may comprise a self-containedtime-delay relay, or other well-known means for producing delayedconduction of a signal, and this delay circuitry may be used ininterrupter circuits having different fault-detection means.

I claim:

1. In a single-phase, two-conductor current supply system wherein a pairof conductors are adapted to supply a current to a load, one of saidconductors being at a high potential with respect to ground, and theother being at a relatively low potential with respect to ground, thecombination comprising:

signal responsive means for interrupting current in the conductor whichis at a high potential with respect to ground, and means responsive to apredetermined change in the potential difference between the otherconductor and ground, resulting from a fault between the conductor athigh potential and ground, for providing a signal to effect interruptionby said interrupting means when said predetermined change occurs. 2. Ina single-phase, two-conductor current supply system wherein a pair ofconductors are adapted to supply a current to a load, one of saidconductors being at a high potential with respect to ground, and theother being at a relatively low otential with respect to ground, thecombination comprising: signal responsive means for interrupting currentin the conductor which is at a high potential with respect to ground,

means connected between the other conductor and ground for conducting acurrent resulting from said relatively low potential difference, and

means for sensing the current in said conducting means and providing asignal to effect interruption by said interrupting means when apredetermined change occurs in the current in said conducting meansresulting from a fault between the conductor at high potential andground.

3. The combination according to claim 2 in which said means for sensingthe current in said conducting means comprises switching means, meansfor effecting opening of said switching means when said predeterminedchange occurs, and a relay having a winding connected to be energizedthrough said switching means and normally closed contacts connected toconduct current to said signal-responsive means.

4. A safety device for preventing electric shock comprising:signal-responsive breaker means for efiecting interruption of current inat least one of a group of conductors, said breaker means includingmanually operable resetting means allowing interruption of current inresponse to a signal even though said resetting means is held,

. means for sensing a ground fault in said one of said conductors andproviding a signal to effect interruption, by said breaker, of currentin said conductor, and

means responsive to a closing of said breaker means for preventing saidsignal from being provided during a period of time immediately followingsaid closing.

5. A safety device according to claim 4, in which said means for sensinga ground fault includes first switching means for conducting said signalto said signal responsive breaker means when a ground fault occurs, andsaid means responsive to a closing of said breaker means includes secondswitching means connected in series with said first switching means andmeans for preventing closing of said second switching means a period oftime immediately following the closing of said breaker means.

6. A safety device according to claim 4, in which said means responsiveto a closing of said breaker means comprises a delayed-action relayhaving contacts connected to conduct said signal to said signalresponsive breaker means.

7. A safety device according to claim 4, in which said means responsiveto a closing of said breaker means comprises a relay having contactsconnected to conduct said signal to said signal responsive breakermeans, said relay having a winding connected through a diode andresistive means to be energized when said breaker means is closed, and acapacitor connected across the winding of said relay.

8. A single-phase, two-conductor current supply system wherein a pair ofconductors are adapted to supply a current to a load, one of saidconductors being at a high potential with respect to ground, and theother being at a relatively low potential with respect to ground, thecombination comprising:

signal-responsive breaker means for interrupting current in theconductor which is at a high potential with respect to ground,

an inductor. connected to conduct current between the ground and theconductor which is at a low potential with respect to ground,

a reed switch,

a magnet arranged in proximity to said reed switch and normally biasingsaid reed switch into a closed condition,

a relay connected to be operated through said reed switch and having aset of normally closed contacts which are held open when said reedswitch is closed,

means connected to said contacts for operating said signal responsivebreaker means when said contacts close upon opening of said reed switch,

said inductor being in proximity to said reed switch and arby saidmagnet when current, resulting from a ground fault, passes through saidinductor.

ranged to overcome the bias imparted to said reed switch UNITED STATESPATENT OFFICE CERTIFICATE OF CORRECTION Patent No. 3, 7 733 Dated Julv I11 1972 Y lnv'ent e) Warren F. Scanlan It is certified that errorappearsin the above-identified patent and that said Letters Patent are herebycorrected as shown below:

7 The, name of the. assignee should be changed to Hazard Sensor Inc.,Rochester, New York.

In column 5, line 47, delete "d iiference".

Signed and sealed this 9th day of January 1973.

(SEAL) Attest:

EDWARD M.FLETCHER,JR. I 1 ROBERT GOTTS CHALK Attesting OfficerCommissioner of Patents FORM PO-1050 (10-69) USCOMM-DC 60376-P69 a u.s.GOVERNMENT PRINTING OFFICE: was o-366-;:u

UNITED STATES PATENT OFFICE CERTIFICATE OF C0 ECTION Pat t 3.676 .738Dated Julv 11 1972 r lnvent e) Warren F. Scanlan It is certified thaterrorappears in the above-identified patent and that said Letters Patentare hereby corrected as shown below:

The name of the. assignee should be changed to Hazard Sensor Inc.Rochester, New York.

In column 5, line 47, delete "difference".

Signed and sealed this 9th day of January l973.

SEAL Attest:

EDWARD M'.FLETCHER,JR. ROBERT GOTTSCHALK v Attesting OfficerCommissioner of Patents FORM FO-105O (10-69) USCOMM-DC 60376-P69 U.S4GOVERNMENT PRINTING OFFICE I 1969 0-366-334

1. In a single-phase, two-conductor current supply system wherein a pairof conductors are adapted to supply a current to a load, one of saidconductors being at a high potential with respect to ground, and theother being at a relatively low potential with respect to ground, thecombination comprising: signal responsive means for interrupting currentin the conductor which is at a high potential with respect to ground,and means responsive to a predetermined change in the potentialdifference between the other conductor and ground, resulting from afault between the conductor at high potential and ground, for providinga signal to effect interruption by said interrupting means when saidpredetermined change occurs.
 2. In a single-phase, two-conductor currentsupply system wherein a pair of conductors are adapted to supply acurrent to a load, one of said conductors being at a high potential withrespect to ground, and the other being at a relatively low potentialwith respect to ground, the combination comprising: signal responsivemeans for interrupting current in the conductor which is at a highpotential with respect to ground, means connected between the otherconductor and ground for conducting a current resulting from saidrelatively low potential difference, and means for sensing the currentin said conducting means and providing a signal to effect interruptionby said interrupting means when a predetermined change occurs in thecurrent in said conducting means resulting from a fault between theconductor at hIgh potential and ground.
 3. The combination according toclaim 2 in which said means for sensing the current in said conductingmeans comprises switching means, means for effecting opening of saidswitching means when said predetermined change occurs, and a relayhaving a winding connected to be energized through said switching meansand normally closed contacts connected to conduct current to saidsignal-responsive means.
 4. A safety device for preventing electricshock comprising: signal-responsive breaker means for effectinginterruption of current in at least one of a group of conductors, saidbreaker means including manually operable resetting means allowinginterruption of current in response to a signal even though saidresetting means is held, means for sensing a ground fault in said one ofsaid conductors and providing a signal to effect interruption, by saidbreaker, of current in said conductor, and means responsive to a closingof said breaker means for preventing said signal from being providedduring a period of time immediately following said closing.
 5. A safetydevice according to claim 4, in which said means for sensing a groundfault includes first switching means for conducting said signal to saidsignal responsive breaker means when a ground fault occurs, and saidmeans responsive to a closing of said breaker means includes secondswitching means connected in series with said first switching means andmeans for preventing closing of said second switching means a period oftime immediately following the closing of said breaker means.
 6. Asafety device according to claim 4, in which said means responsive to aclosing of said breaker means comprises a delayed-action relay havingcontacts connected to conduct said signal to said signal responsivebreaker means.
 7. A safety device according to claim 4, in which saidmeans responsive to a closing of said breaker means comprises a relayhaving contacts connected to conduct said signal to said signalresponsive breaker means, said relay having a winding connected througha diode and resistive means to be energized when said breaker means isclosed, and a capacitor connected across the winding of said relay.
 8. Asingle-phase, two-conductor current supply system wherein a pair ofconductors are adapted to supply a current to a load, one of saidconductors being at a high potential with respect to ground, and theother being at a relatively low potential with respect to ground, thecombination comprising: signal-responsive breaker means for interruptingcurrent in the conductor which is at a high potential with respect toground, an inductor connected to conduct current between the ground andthe conductor which is at a low potential with respect to ground, a reedswitch, a magnet arranged in proximity to said reed switch and normallybiasing said reed switch into a closed condition, a relay connected tobe operated through said reed switch and having a set of normally closedcontacts which are held open when said reed switch is closed, meansconnected to said contacts for operating said signal responsive breakermeans when said contacts close upon opening of said reed switch, saidinductor being in proximity to said reed switch and arranged to overcomethe bias imparted to said reed switch by said magnet when current,resulting from a ground fault, passes through said inductor.